---
name: stm32f103c8t6-flashing
user-invocable: true
description: "Use when programming an STM32F103C8T6: flashing over SWD with OpenOCD or ST-Link, entering the ROM UART bootloader with BOOT0, why USB DFU does not work on a bare F103, and clearing readout protection. Triggers: flash the stm32, program blue pill, stm32 bootloader, BOOT0 flashing, openocd stm32, st-link, stm32 dfu not working."
---

# STM32F103C8T6 flashing

Two hardware paths exist on a bare F103: SWD, and the ROM UART bootloader.
USB is NOT one of them (see the gotcha below).

## SWD (preferred)

Wire SWDIO (PA13, pin 34), SWCLK (PA14, pin 37), NRST, 3V3, GND to an
ST-Link or compatible probe, then:

```
openocd -f interface/stlink.cfg -f target/stm32f1x.cfg \
  -c "program firmware.elf verify reset exit"
```

Works regardless of BOOT0 and does not disturb running clocks. If the chip
was put into a deep-sleep loop by bad firmware, hold NRST low while OpenOCD
connects (connect-under-reset).

## ROM UART bootloader

Strap BOOT0 high, reset, and the ROM bootloader listens on USART1: PA9 (TX)
and PA10 (RX), 3.3 V logic, even parity. Flash with stm32flash or
STM32CubeProgrammer, then strap BOOT0 back low and reset. Useful when no
debug probe is available; needs only a USB-UART adapter.

## The USB gotcha

The F103's ROM bootloader speaks USART1 ONLY. A bare F103 cannot be flashed
over USB: no ROM DFU exists on this family (that arrived on later parts).
Boards that flash over USB (many Blue Pill variants) do it through a
SOFTWARE bootloader (stm32duino/Maple) that someone previously flashed via
SWD or UART. If USB flashing "stopped working", the software bootloader was
erased: recover over SWD or the UART ROM loader.

## Readout protection

If a previous firmware set RDP, SWD reads fail and programming errors out.
Clear it (this MASS-ERASES flash):

```
openocd -f interface/stlink.cfg -f target/stm32f1x.cfg \
  -c "init; reset halt; stm32f1x unlock 0; reset run; exit"
```

Power-cycle after unlocking; RDP state latches until a full reset.